1. Field of the Invention
This invention relates to a light beam scanning apparatus used for image forming apparatus and systems such as a duplication machine, printer, facsimile apparatus and other similar systems, and more particularly to such scanning apparatus provided with the capability of reducing a change in the diameter of beam spots on a photoreceptor while scanning light beams thereon.
2. Discussion of the Background
In the course of recent developments of digital duplication machines and laser printers, more attention has been focused on the increase in the density of recording, which necessitates the reduction of the diameter of beam spots formed on a photoreceptor in these machines. In order to achieve such improvements, several disclosures have been put forward such as, for example, an optical unit having so-called free-form curvature to effectively correct aberration, in which the unit is formed to have a curvature without rotational axis.
As an example, Japanese Laid-Open Patent Application No. 10-54952 discloses a light beam scanning apparatus which includes first and second anamorphic optical elements. In the scanning apparatus in that disclosure, the magnification in the horizontal scanning direction of the first anamorphic optical element is designed to be varied with the position in the horizontal direction such that the magnification of the light beam scanning apparatus, consisting of the first and second anamorphic optical elements, is constant irrespective of the deflection angle. As a result, an undesirable change in beam spot diameter due to large image height in the vertical scanning direction can be alleviated along with undue changes in the distance between neighboring scanning lines in the case of multiple-beam scanning apparatus.
In such a light beam scanning apparatus, however, several difficulties arise as to maintaining sufficient accuracy of fabricating optical elements and assembling the scanning apparatus due to possible errors during fabrication or assembly steps, when the magnification of the light beam scanning apparatus changes with deflection angle.
Accordingly, it is an object of the present invention to provide a light beam scanning apparatus, having most, if not all, of the advantages and features of similar employed systems, while eliminating many of the aforementioned disadvantages.
It is another object to provide a light beam scanning apparatus in which the magnification of the light beam scanning apparatus in the vertical scanning direction is maintained to be approximately constant irrespective of deflection angle. This becomes feasible by making the power of a first anamorphic optical element L1 in the vertical scanning direction approximately constant irrespective of deflection angle and, with the thus fabricated optical elements and scanning apparatus, by alleviating undesirable change of beam spot diameter in the vertical scanning direction, which is caused by a large image height, yet reducing undue increase in aberration caused by possible errors during fabrication or assembly steps.
The following brief description is a synopsis of only selected features and attributes of the present invention. A more complete description thereof is found below in the section entitled xe2x80x9cDescription of the Preferred Embodiments.xe2x80x9d
The light beam scanning apparatus disclosed herein includes at least a light source for emitting light beams, a first optical unit is configured to optically couple together the light beams emitted from the light source, a second optical unit is configured to converge the light beams received from the first optical unit into an approximately line image elongated along the main (i.e., horizontal) scanning direction, a light beam deflector is configured to deflect the converged light beams from the second optical unit by intersection with reflecting planes situated at least at the vicinity of the position of formation of the line image, and a third optical unit is configured to converge the light beams deflected by the light beam deflector into a spot formed on the surface to be scanned, in which the third optical unit includes of a first anamorphic optical element, L1, and a second anamorphic optical element, L2, each having a positive power along the horizontal direction and the vertical scanning directions, respectively, and the magnification of the third optical unit is made approximately constant irrespective of deflection angle and the power of the first anamorphic optical element L1 is also made approximately constant irrespective of deflection angle.
In addition, the light beam scanning apparatus is adjusted to satisfy the relation, 0xe2x89xa6xcex94P1/xcex94P2 less than 0.1, where P1 and P2 are powers in the vertical scanning direction for the first anamorphic optical element L1 and the second anamorphic optical element L2, respectively, and xcex94P1 and xcex94P2 are the differences between the maximum and minimum values of P1 and P2, respectively.
In another aspect of the invention, the curvature of the cross section of the first anamorphic optical element L1 included in the light beam scanning apparatus varies independently of the shape thereof in the horizontal scanning direction. In addition, the curvature of the cross section of the second anamorphic optical element L2 varies independently of the shape thereof in the horizontal scanning direction.
In yet another aspect of the invention, the second anamorphic optical element L2 is formed having a specified surface shape with the curvature of its cross section depending on the position in the vertical scanning direction such that the magnification of the third optical unit is approximately constant irrespective of deflection angle.
In another aspect of the invention, at least one of the first and second anamorphic optical elements, L1 and L2, is formed with a specified surface shape such that the curvature of its cross section in the vertical scanning direction varies independently of its shape in the horizontal scanning direction and that the shape varies asymmetrically depending on the position in the horizontal scanning direction.
In addition, at least one of surfaces of the first and second anamorphic optical elements, L1 and L2, is formed to be in the form of noncircular arc.
In another aspect of the invention, the lateral magnification, xcex2(0), in the vertical scanning direction on the optical axis of the third optical unit satisfies the relation, 0.5 less than |xcex2(0)| less than 1.5. In addition, lateral magnifications, xcex2(0) and xcex2(xcex8), in the vertical scanning direction of the third optical unit on the optical axis and at a deflection angle, xcex8, from the optical axis, respectively, satisfy the relation, 0.8 less than |xcex2(xcex8)/xcex2(0)| less than 1.2.
In another aspect, the light source may be a multiple-beam light source which emits multiple fluxes of light beams.
An image forming system is also disclosed herein as incorporating anyone of the optical units recited herein above.
Further aspects of the present invention and the manner in which it addresses the above problems, as well as others, will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings, detailed description, and claims.